Machine Structural Steel Material Having Low Heat-Treatment Deformation

ABSTRACT

There is provided a steel material with small heat treatment deformation, comprising a machine structural steel used for components for power transmission, such as gears and shafts used in automobiles, industrial machines, and the like. In the steel material, the Ms point of the steel material comprising a machine structural steel, comprising in mass %: C: 0.20 to 0.30%; Si: 0.10 to 1.50%; Mn: 0.10 to 1.20%; P: 0.030% or less; S: 0.030% or less; Cr: 1.30 to 2.50%; Cu: 0.30% or less; Al: 0.008 to 0.300%; O: 0.0030% or less; and N: 0.0020 to 0.0300%; and the balance Fe and unavoidable impurities, is 460° C. or less; a ratio (J9/J1.5) of hardness J9 at a distance of 9 mm from the quenched end of the steel material to hardness J1.5 at a distance of 1.5 mm from the quenched end of the steel material, as measured by Jominy end quenching method, is in a range of from 0.70 to 0.85; and a ratio (J11/J1.5) of hardness J11 at a distance of 11 mm from the quenched end of the steel material to hardness J1.5 is in a range of from 0.67 to 0.78.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2012-193763 filed on Sep. 4, 2012, the entire content of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to machine structural steels, for example, used for power transmission components such as gears and shafts used in automobiles, industrial machines, and the like, and particularly relates to a machine structural steel with small heat treatment deformation.

BACKGROUND ART

It is known that deformation of a steel material occurs due to heat treatment such as quenching (hereinafter referred to as “heat treatment deformation”). Such heat treatment deformation has such an adverse effect that the number of production steps is increased in order to correct the deformation, the percentage of defective components is increased when the deformation is too large to correct, or noise or vibration is caused by the deformation in the case where a deformed steel material is incorporated as a driving system component. Accordingly, minimizing the heat treatment deformation is a very important problem with regard to the practical use.

Conventionally, the heat treatment deformation has been considered to be also affected by a large number of factors such as a component shape, the effect of a step prior to heat treatment, the physical property value of a refrigerant such as a quenching oil, and the non-uniformity of cooling, as factors other than a steel material. Thus, reduction in heat treatment deformation has been attempted by variously adjusting these factors to be adequate. For example, there has been proposed, as a material measure, a method of precipitating a soft ferrite phase in the core of a quenched steel material to reduce heat treatment distortion (e.g., see Patent Literature 1).

As an approach from a cooling method, there has been proposed a method of utilizing pressurized gas cooling instead of conventional oil quenching (e.g., see Patent Literature 2). Further, there has been proposed a method of attempting uniform cooling of a substance to be cooled using means for promoting or decreasing a heat transfer coefficient (e.g., see Patent Literature 3).

In addition, in Patent Literature 3, the means for promoting a heat transfer coefficient is considered to be due to a coating material for promoting cooling that is placed in a site where cooling is delayed or due to convection of a coolant formed around the site where cooling is delayed, and the means for decreasing a heat transfer coefficient is considered to be due to glass wool or a heat insulating coating material which covers a site where cooling is easy to proceed.

CITATION LIST Patent Literature [Patent Literature 1] Japanese Patent Laid-Open Publication No. 1997-111408 [Patent Literature 2] Japanese Patent Laid-Open Publication No. 2008-121064 [Patent Literature 3] Japanese Patent Laid-Open Publication No. 2010-174289 SUMMARY OF INVENTION

However, the conventional methods proposed as described above have not always been able to be general-purpose means. This is because, for example, the technology of Patent Literature 1 may involve introduction of the soft phase with low strength into a component; the technology of Patent Literature 2 requires a modification of a heat treating furnace itself; and the technology of Patent Literature 3 requires treatment to an individual component to be heat-treated, so that it has been difficult to consider that the conventional methods are general-purpose means.

In contrast, the inventors have extensively researched a steel of which the heat treatment deformation can be reduced to a low level even when cooling of a component is non-uniform under a common technique such as oil quenching, while securing the sufficient strength of the steel material without relying on generation of a ferrite morphology which is soft and may result in reduced component strength. As a result, the inventors have found that the heat treatment deformation can be reduced to a low level by suitably controlling the chemical constituents of the steel, martensitic transformation start temperature (Ms point), and hardenability as measured by Jominy end quenching method in appropriate ranges.

It is therefore an object of the present invention to provide a steel material with small heat treatment deformation, comprising a machine structural steel used for components for power transmission, such as gears and shafts used in automobiles, industrial machines, and the like.

According to an embodiment of the present invention, there is provided a machine structural steel material with small heat treatment deformation, the steel material being a machine structural steel comprising in mass %:

-   -   C: 0.20 to 0.30%;     -   Si: 0.10 to 1.50%;     -   Mn: 0.10 to 1.20%;     -   P: 0.030% or less;     -   S: 0.030% or less;     -   Cr: 1.30 to 2.50%;     -   Cu: 0.30% or less;     -   Al: 0.008 to 0.300%;     -   O: 0.0030% or less;     -   N: 0.0020 to 0.0300%; and     -   the balance Fe and unavoidable impurities,     -   wherein the steel material comprising the steel has a         martensitic transformation start temperature (Ms point) of         460° C. or less;     -   a value of (J9/J1.5) calculated by following equation (1) using         hardness J1.5 at a distance of 1.5 mm and hardness J9 at a         distance of 9 mm from a quenched end of the steel material, as         measured by Jominy end quenching method for the steel material,         is in a range of from 0.70 to 0.85; and     -   a value of (J11/J1.5) calculated by following equation (2) using         hardness J1.5 at a distance of 1.5 mm and hardness J11 at a         distance of 11 mm is in a range of from 0.67 to 0.78:

(J9/J1.5)=(hardness at distance of 9 mm from a quenched end, as measured by Jominy end quenching method)/(hardness at distance of 1.5 mm from a quenched end, as measured by Jominy end quenching method)  Equation (1); and

(J11/J1.5)=(hardness at distance of 11 mm from a quenched end, as measured by Jominy end quenching method)/(hardness at distance of 1.5 mm from a quenched end, as measured by Jominy end quenching method)  Equation (2).

According to one preferred embodiment of the present invention, there is provided a machine structural steel material with small heat treatment deformation, the steel material being a steel material comprising a machine structural steel comprising in mass %:

-   -   C: 0.20 to 0.30%;     -   Si: 0.10 to 1.50%;     -   Mn: 0.10 to 1.20%;     -   P: 0.030% or less;     -   S: 0.030% or less;     -   Cr: 1.30 to 2.50%;     -   Cu: 0.30% or less;     -   Al: 0.008 to 0.300%;     -   O: 0.0030% or less;     -   N: 0.0020 to 0.0300%;     -   one or two of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.50%; and     -   the balance Fe and unavoidable impurities,     -   wherein the steel material has a martensitic transformation         start temperature (Ms point) of 460° C. or less;     -   a value of (J9/J1.5) calculated by equation (1) described above         using hardness J1.5 at a distance of 1.5 mm and hardness J9 at a         distance of 9 mm from a quenched end of the steel material, as         measured by Jominy end quenching method for the steel material,         is in a range of from 0.70 to 0.85; and     -   a value of (J11/J1.5) calculated by equation (2) described above         using hardness J1.5 at a distance of 1.5 mm and hardness J11 at         a distance of 11 mm is in a range of from 0.67 to 0.78.

According to another embodiment of the present invention, there is provided a machine structural steel material with small heat treatment deformation, the steel material being a steel material comprising a machine structural steel comprising in mass %:

-   -   C: 0.20 to 0.30%;     -   Si: 0.10 to 1.50%;     -   Mn: 0.10 to 1.20%;     -   P: 0.030% or less;     -   S: 0.030% or less;     -   Cr: 1.30 to 2.50%;     -   Cu: 0.30% or less;     -   Al: 0.008 to 0.300%;     -   O: 0.0030% or less;     -   N: 0.0020 to 0.0300%;     -   one or two of Ti: 0.020 to 0.200% and Nb: 0.02 to 0.20%; and     -   the balance Fe and unavoidable impurities,     -   wherein the steel material has a martensitic transformation         start temperature (Ms point) of 460° C. or less;     -   a value of (J9/J1.5) calculated by equation (1) described above         using hardness J1.5 at a distance of 1.5 mm and hardness J9 at a         distance of 9 mm from a quenched end of the steel material, as         measured by Jominy end quenching method for the steel material,         is in a range of from 0.70 to 0.85; and     -   a value of (J11/J1.5) calculated by equation (2) described above         using hardness J1.5 at a distance of 1.5 mm and hardness J11 at         a distance of 11 mm is in a range of from 0.67 to 0.78.

According to another embodiment of the present invention, there is provided a machine structural steel material with small heat treatment deformation, the steel material being a steel material comprising a machine structural steel comprising in mass %:

-   -   C: 0.20 to 0.30%;     -   Si: 0.10 to 1.50%;     -   Mn: 0.10 to 1.20%;     -   P: 0.030% or less;     -   S: 0.030% or less;     -   Cr: 1.30 to 2.50%;     -   Cu: 0.30% or less;     -   Al: 0.008 to 0.300%;     -   O: 0.0030% or less;     -   N: 0.0020 to 0.0300%;     -   one or two of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.50%;     -   one or two of Ti: 0.020 to 0.200% and Nb: 0.02 to 0.20%; and     -   the balance Fe and unavoidable impurities,     -   wherein the steel material has a martensitic transformation         start temperature (Ms point) of 460° C. or less;     -   a value of (J9/J1.5) calculated by equation (1) described above         using hardness J1.5 at a distance of 1.5 mm and hardness J9 at a         distance of 9 mm from a quenched end of the steel material, as         measured by Jominy end quenching method for the steel material,         is in a range of from 0.70 to 0.85; and     -   a value of (J11/J1.5) calculated by equation (2) described above         using hardness J1.5 at a distance of 1.5 mm and hardness J11 at         a distance of 11 mm is in a range of from 0.67 to 0.78.

According to another embodiment of the present invention, there is provided a machine structural steel material with small heat treatment deformation, the steel material comprising in mass %:

-   -   C: 0.20 to 0.30%;     -   Si: 0.10 to 1.50%;     -   Mn: 0.10 to 1.20%;     -   P: 0.030% or less;     -   S: 0.030% or less;     -   Cr: 1.30 to 2.50%;     -   Cu: 0.30% or less;     -   Al: 0.008 to 0.300%;     -   O: 0.0030% or less;     -   N: 0.0020 to 0.0300%;     -   Ni: 0 to 3.00%;     -   Mo: 0 to 0.50%;     -   Ti: 0 to 0.200%; and     -   Nb: 0 to 0.20%; and     -   the balance Fe and unavoidable impurities,     -   wherein the steel material has a martensitic transformation         start temperature (Ms point) of 460° C. or less;     -   a ratio (J9/J1.5) of hardness J9 at a distance of 9 mm from a         quenched end of the steel material to hardness J1.5 at a         distance of 1.5 mm from a quenched end of the steel material, as         measured by Jominy end quenching method, ranges from 0.70 to         0.85; and     -   a ratio (J11/J1.5) of hardness J11 at a distance of 11 mm from a         quenched end of the steel material to hardness J1.5 at a         distance of 1.5 mm from a quenched end of the steel material         ranges from 0.67 to 0.78.

According to one preferred embodiment of the present invention, the above-described steel material is essentially free of Ni, Mo, Ti, and Nb, or comprises them at unavoidable impurity levels.

According to one preferred embodiment of the present invention, the above-described steel material comprises, in mass %, one or two of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.50%.

According to one preferred embodiment of the present invention, the above-described steel material comprises, in mass %, one or two of Ti: 0.020 to 0.200% and Nb: 0.02 to 0.20%.

According to one preferred embodiment of the present invention, the above-described steel material comprises, in mass %, one or two of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.50% and one or two of Ti: 0.020 to 0.200% and Nb: 0.02 to 0.20%.

DESCRIPTION OF EMBODIMENTS

The present invention is specifically explained below. In addition, “%” of each constituent shows mass %.

The machine structural steel material with small heat treatment deformation according to the present invention comprises in mass %: C: 0.20 to 0.30%; Si: 0.10 to 1.50%; Mn: 0.10 to 1.20%; P: 0.030% or less; S: 0.030% or less; Cr: 1.30 to 2.50%; Cu: 0.30% or less; Al: 0.008 to 0.300%; O: 0.0030% or less; N: 0.0020 to 0.0300%; Ni: 0 to 3.00%; Mo: 0 to 0.50%; Ti: 0 to 0.200%; and Nb: 0 to 0.20%; and the balance Fe and unavoidable impurities, preferably consists essentially of these elements and unavoidable impurities, and more preferably consists of these elements and unavoidable impurities.

C: 0.20 to 0.30%

C is an element necessary for securing the strength of the steel material after quenching and tempering thereof or the strength of its core after carburizing, quenching, and tempering thereof for a machine structural component, and adjustment of C content into a specified range is needed for reducing heat treatment deformation. A content of C of less than 0.20% fails to secure the strength, while that of more than 0.30% results in too large heat treatment deformation. Thus, the content of C is set at 0.20 to 0.30%, desirably at 0.22 to 0.27%.

Si: 0.10 to 1.50%

Si is an element that is necessary for deoxidation and that is effective for imparting strength and hardenability required for steel. However, a content of Si of less than 0.10% fails to give the effects, while that of more than 1.50% results in deteriorated mechanical workability. Thus, the content of Si is set at 0.10 to 1.50%, desirably at 0.20 to 1.00%.

Mn: 0.10 to 1.20%

Mn is an element necessary for securing hardenability. However, a content of Mn of less than 0.10% fails to provide a sufficient effect for hardenability, while that of more than 1.20% results in deteriorated mechanical workability. Thus, the content of Mn is set at 0.10 to 1.20%, desirably at 0.20 to 0.80%, more desirably at 0.20 to 0.55%.

P: 0.030% or less

P is an unavoidable element that is incorporated from scrap but results in grain boundary segregation to deteriorate characteristics such as impact strength and bending strength. Thus, the content of P is set at 0.030% or less (including 0%), typically at more than 0 and 0.030% or less.

S: 0.030% or less

S is an element that improves machinability but generates MnS, which is a non-metallic inclusion to deteriorate crosswise toughness and fatigue strength. Thus, the content of S is set at 0.030% or less (including 0%), typically at more than 0 and 0.030% or less.

Cr: 1.30 to 2.50%

Cr is an element necessary for securing hardenability. However, a content of Cr of less than 1.30% results in an insufficient effect for hardenability, while that of more than 2.50% results in inhibited carburization and also in deteriorated mechanical workability. Thus, the content of Cr is set at 1.30 to 2.50%, desirably at 1.50 to 2.25%.

Ni: 0.20 to 3.00%

Ni is an optional element that improves hardenability and toughness and addition of 0.20% or more thereof is preferred for obtaining the effect. However, since a content of Ni of more than 3.00% significantly deteriorates workability and increases a cost, the content of Ni is set at 3.00% or less. Thus, the content of Ni is set at 0.20 to 3.00%.

Mo: 0.05 to 0.50%

Mo is an optional element that improves hardenability and toughness, and addition of 0.05% or more thereof is preferred for obtaining the effect. However, a content of Mo of more than 0.50% deteriorates workability. Thus, the content of Mo is set at 0.05 to 0.50%.

Cu: 0.30% or less

Cu is an unavoidable element that is incorporated from scrap, but has an aging property and is effective at increasing strength. However, a content of Cu of more than 0.30% results in deteriorated hot workability. Thus, the content of Cu is set at 0.30% or less (including 0%), typically at more than 0 and 0.30% or less.

Al: 0.008 to 0.300%

Al is an element that is used as a deoxidation material and is bound to N to be precipitated as AlN to result in the effect of suppressing coarsening of grain size, as described below. Addition of 0.008% or more of Al is necessary for obtaining the effect. In contrast, addition of more than 0.300% of Al results in the formation of large-sized alumina-based inclusions, and deteriorates fatigue characteristics and workability. Thus, the content of Al is set at 0.008 to 0.300%, desirably at 0.014 to 0.200%.

O: 0.0030% or less

O is an element that is unavoidably contained in steel. However, an O content of more than 0.0030% results in the deterioration of workability and fatigue strength due to the increase of oxides. Thus, the content of 0 is set at 0.0030% or less (including 0%), desirably at 0.0020% or less (including 0%). Further, the content of 0 is set typically at more than 0 and 0.0030% or less, desirably at more than 0 and 0.0020% or less.

N: 0.0020 to 0.0300%

N is an element that is finely precipitated as AlN and Nb nitrides in steel and provides the effect of preventing coarsening of grain size, and addition of 0.0020% or more thereof is necessary for obtaining the effect. However, a content of N of more than 0.0300% results in the increase of the nitrides and deteriorates fatigue strength and workability. Thus, the content of N is set at 0.0020 to 0.0300%, desirably at 0.0020 to 0.0200%. Particularly in the steel that contains 0.020% or more of Ti, the content of N is set at 0.0020 to 0.0100% in order to avoid the deterioration of fatigue strength due to the excessive generation of TiN.

Ti: 0.020 to 0.200%

Ti is an optional element that is bound to C in steel to finely form a carbide and provides the effect of preventing coarsening of grain size, and addition of 0.020% or more of Ti is preferred for obtaining the effect. In contrast, a content of Ti of more than 0.200% results in deteriorated mechanical workability. Thus, the content of Ti is set at 0.020 to 0.200%.

Nb: 0.02 to 0.20%

Nb is an optional element that forms a carbide or a nitride and provides the effect of preventing coarsening of grain size. In particular, NbC or Nb(C, N) with a nanometer-order size, which is finely dispersed in steel, suppresses the growth of the grain size. A content of Nb of less than 0.02% prevents the effect from being obtained while that of more than 0.20% results in the excessive amount of a precipitate to deteriorate workability. Thus, the content of Nb is set at 0.02 to 0.20%, desirably at 0.02 to 0.12%.

Furthermore, the reasons of the limitation of an Ms point and hardenability measured by Jominy end quenching method, other than the reasons of the limitation of the above-described constituents, will be explained.

Ms Point: 460° C. or Less

In the steel material according to the present invention, it is necessary to regulate its martensitic transformation start temperature (Ms point) to 460° C. or less in order to reduce the heat treatment deformation of the steel material. The reason why the heat treatment deformation can be reduced by regulating the Ms point to 460° C. or less is that, during quenching, even when cooling of a component is non-uniform, occurrence of martensitic transformation can be avoided in a temperature range in which the cooling performance of a refrigerant is high and, as a result, a time point at which the martensitic transformation occurs can be inhibited from greatly differing depending on the site of the component. Thus, the Ms point is regulated to 460° C. or less, and the Ms point is desirably regulated to 450° C. or less. The heat treatment deformation in this case refers to bending of a shaft-shaped component or inclination and/or torsion of a gear tooth from a designed shape.

Value of (J9/J1.5): 0.70 to 0.85 Value of (J11/J1.5): 0.67 to 0.78

When a ratio (J9/J1.5) of hardness J9 at a distance of 9 mm from the quenched end of the steel material to hardness J1.5 at a distance of 1.5 mm from the quenched end of the steel material, as measured by Jominy end quenching method, is in a range of 0.70 to 0.85 and a ratio (J11/J1.5) of hardness J11 at a distance of 11 mm from the quenched end of the steel material to hardness J1.5 at a distance of 1.5 mm from the quenched end of the steel material is in a range of 0.67 to 0.78, the heat treatment deformation of the steel material can be suppressed to a low level. The heat treatment deformation in this case refers to bending of a shaft-shaped component or inclination and/or torsion of a gear tooth from a designed shape after quenching or to a variation in dimension (length, diameter, thickness, or the like) of a component before and after quenching. Although a mechanism to suppress the heat treatment deformation by controlling Jominy hardenability in an appropriate range has not yet been able to be sufficiently elucidated, it has been experimentally confirmed by the inventors that the heat treatment deformation becomes large even when the Jominy hardenability is too low or too high. Without being bound by theory, it is believed that in a steel having Jominy hardenability in the range, bainitic transformation moderately occurs prior to martensitic transformation in a cooling process of quenching; the strength of a steel material is increased; the martensitic transformation starts from a state in which deformation is inhibited from occurring in some degree; and heat treatment deformation is therefore suppressed. In contrast, it is believed that heat treatment deformation is increased by the influence of the bainitic transformation in itself due to an excessive bainitic transformation, when hardenability is too low, and heat treatment deformation is also increased as a bainitic structure that reduces heat treatment deformation is small, when hardenability is too high.

The limitation of the steel constituents, the limitation of the Ms point, and the limitation of hardenability measured by the Jominy end quenching method, as mentioned above, achieves the smaller heat treatment deformation in the case of processing a steel material into a component and then performing quenching or carburizing and quenching for hardening the component. As a result, the present invention can provide such a beneficial effect of expecting the improvement of the yield of components, the simplification or removal of the step of correcting a component, or the omission of the grinding of a gear tooth surface for measures against noise and vibration.

EXAMPLES

The steel material according to the present invention is specifically explained with reference to Examples below.

In order to obtain the machine structural steel used as components for power transmission, such as gears and shafts used in automobiles, industrial machines, and the like, steels comprising compositions of present invention examples Nos. 1 to 23 shown in Table 1 and the balance Fe and unavoidable impurities were ingotted in a vacuum induction melting furnace to obtain 100 kg of steel ingots.

TABLE 1 (unit: mass %) No. C Si Mn P S Ni Cr Mo Cu Al O N Others Present 1 0.22 0.22 0.75 0.012 0.014 0.02 1.72 0.01 0.08 0.031 0.0010 0.0148 — Invention 2 0.22 0.54 0.32 0.010 0.008 0.06 1.86 0.02 0.05 0.034 0.0009 0.0155 — Examples 3 0.24 0.55 0.56 0.012 0.009 0.12 1.35 0.01 0.11 0.028 0.0008 0.0165 — 4 0.27 0.30 0.85 0.009 0.011 0.11 1.45 0.02 0.09 0.019 0.0007 0.0138 — 5 0.28 0.19 0.35 0.012 0.008 0.08 1.55 0.02 0.05 0.082 0.0006 0.0128 — 6 0.21 0.82 0.45 0.018 0.012 0.10 1.65 0.01 0.06 0.032 0.0006 0.0160 — 7 0.20 0.15 0.38 0.015 0.015 0.54 2.23 0.02 0.11 0.028 0.0008 0.0135 — 8 0.23 0.20 0.45 0.011 0.014 0.85 1.34 0.02 0.10 0.025 0.0011 0.0144 — 9 0.25 0.20 0.26 0.007 0.008 1.24 1.42 0.01 0.11 0.040 0.0009 0.0166 — 10 0.26 0.48 0.36 0.008 0.009 0.09 1.50 0.06 0.07 0.022 0.0012 0.0148 — 11 0.29 0.56 0.50 0.008 0.011 0.03 1.35 0.15 0.06 0.031 0.0008 0.0135 — 12 0.30 0.30 0.20 0.011 0.008 0.14 1.51 0.25 0.11 0.017 0.0006 0.0136 — 13 0.28 0.31 0.45 0.012 0.013 0.75 1.32 0.22 0.06 0.025 0.0006 0.0128 — 14 0.24 0.22 0.41 0.009 0.013 0.06 2.32 0.01 0.03 0.030 0.0007 0.0142 0.04% Nb 15 0.25 0.54 0.28 0.014 0.015 0.10 1.85 0.01 0.11 0.030 0.0006 0.0120 0.04% Nb 16 0.25 0.29 0.33 0.015 0.008 0.08 1.84 0.02 0.05 0.050 0.0010 0.0065 0.035% Ti 0.06% Nb 17 0.22 0.47 0.41 0.008 0.009 0.05 1.91 0.01 0.10 0.041 0.0012 0.0055 0.110% Ti 18 0.28 0.51 0.78 0.009 0.007 0.04 1.35 0.02 0.12 0.019 0.0007 0.0062 0.156% Ti 19 0.23 0.60 0.29 0.007 0.012 0.44 1.61 0.01 0.12 0.026 0.0008 0.0188 0.03% Nb 20 0.22 0.31 0.67 0.012 0.013 0.86 1.42 0.02 0.13 0.026 0.0006 0.0173 0.04% Nb 21 0.24 0.35 0.32 0.013 0.012 2.22 1.62 0.02 0.08 0.018 0.0006 0.0052 0.026% Ti 0.08% Nb 22 0.26 0.55 0.30 0.007 0.012 0.08 1.91 0.09 0.05 0.028 0.0007 0.0049 0.123% Ti 23 0.29 0.80 0.22 0.008 0.007 0.04 1.32 0.25 0.05 0.031 0.0008 0.0033 0.180% Ti 0.20% or less Ni and 0.05% or less Mo are unavoidable impurities.

In the same manner as in the present invention examples described above, as the machine structural steel used for components for power transmission, such as gears and shafts used in automobiles, industrial machines, and the like, steels comprising compositions of comparative examples Nos. 1 to 16 shown in Table 2 and the balance Fe and unavoidable impurities were ingotted in the vacuum induction melting furnace to obtain 100 kg of steel ingots.

TABLE 2

(unit. mass %) (The shaded portions fall outside the scope of claims. 0.20% or less Ni and 0.05% or less Mo are unavoidable impurities.)

First, the steel ingots, ingotted as described above, of the present invention examples and the comparative examples were heated at 1250° C. for 5 hours and then forged to obtain steel bars having a diameter of 32 mm. Then, the steel bars were normalized by heating and maintaining at 900° C. for 1.5 hours, followed by air-cooling. Subsequently, test pieces having a diameter of 20 mm and a length of 80 mm were produced from the steel bars having the diameter of 32 mm, and the sides of the test pieces were subjected to processing to have a groove with a depth of 5 mm, a width of 8 mm, and a length of 80 mm. The groove processing caused a cooling rate to greatly differ depending on a site in each test piece during quenching. Further, the lengths of the test pieces after the groove processing were measured. In addition, a radius and a groove width were measured at each position of 2 mm and 20 mm from each test piece end and 40 mm which is the center of each test piece length. Subsequently, the test pieces were carburized at 930° C., their temperatures were then decreased to 850° C. in the furnace, and they were further maintained for 1 hour and then quenched into quenching oil at 60° C. After the quenching, as for the sufficiently cooled test pieces, the bends and lengths of the test pieces and a radius and a groove width at each position of 2 mm and 20 mm from each test piece end and 40 mm which is the center of each test piece length were measured.

Each bend after the heat treatment was determined by holding both ends of each test piece with V blocks, measuring the maximum and minimum displacements on the circumference of the central portion of the test piece during one revolution of the test piece, with a dial gauge, and dividing the difference between the maximum and minimum displacements by 2. In the case of this measurement, the displacement at the bottom of a groove present on the circumference of the test piece was ignored. Further, the difference between the lengths of the test piece before and after the heat treatment was determined to evaluate the absolute value thereof as the index of heat treatment deformation. Furthermore, the dimensional change amounts of the radius and the groove width before and after the heat treatment at each place were determined based on the dimensional measurement results of the radius and the groove width before and after the heat treatment at each position of three places in total of 2 mm and 20 mm from each test piece end and 40 mm which is the center of each test piece length, and values obtained by subtracting the minimum values from the maximum values of the dimensional change amounts at the three places were then defined as a radius change amount and a groove width change amount, respectively, and evaluated as indices for heat treatment deformation.

Further, test pieces having a diameter of 3 mm and a length of 10 mm were extracted from the steel bars having the diameter of 32 mm after subjected to the normalizing described above to measure Ms points as the martensitic transformation start temperatures of the steel materials using a fully-automated transformation record measuring apparatus. The Ms points in the present embodiment were to be measured under the conditions in which the process of cooling a component was presumed. Therefore, in the present embodiment, the Ms points were measured at a cooling rate of 30° C./s during the quenching by simulating the case where the above-described test pieces with grooves having the diameter of 20 mm were oil quenched at an oil temperature of 60° C. For the measurement of the hardenability of the steel materials by the Jominy end quenching method, test pieces were produced from the above-described forged steel bars having the diameter of 32 mm, and were tested and evaluated under the conditions according to “method of hardenability test for steel” (end quenching method) specified in JIS G 0561.

In Table 3, there are shown the measured Ms points measured for the steels of the present invention examples, each value of hardness J1.5 at a distance of 1.5 mm from the quenched ends, hardness J9 at a distance of 9 mm, and hardness J11 at a distance of 11 mm, measured by the Jominy end quenching method, and the determined values of (J9/J1.5) and (J11/J1.5). The bends (unit: mm) evaluated after quenching the above-described test pieces and the absolute values (unit: mm) of the differences between the lengths of the test pieces before and after the heat treatment, and the radius change amounts (unit: mm) and groove width change amounts (unit: mm) of the test pieces before and after the heat treatment, determined by the above-described method, are further shown. In the steel materials of the present invention examples Nos. 1 to 23, as shown in Table 3, the martensitic transformation start temperatures, i.e., the Ms points were in a range of 388 to 444° C.; the values of equation (1), shown as follows, of (J9/J1.5) of the steel materials were in a range of 0.72 to 0.85; the values of equation (2), shown as follows, of (J11/J1.5) were in a range of 0.67 to 0.78; the bends after the heat treatment were 0.005 to 0.030 mm; the absolute values of the differences between the lengths of the test pieces before and after the heat treatment were 0.003 to 0.023 mm; the radius change amounts before and after the heat treatment were 0.002 to 0.008 mm; and the groove width change amounts before and after the heat treatment were 0.011 to 0.024 mm.

(J9/J1.5)=(hardness at distance of 9 mm from quenched end, measured by Jominy end quenching method)/(hardness at distance of 1.5 mm from quenched end, measured by Jominy end quenching method)  (1)

(J11/J1.5)=(hardness at distance of 11 mm from quenched end, measured by Jominy end quenching method)/(hardness at distance of 1.5 mm from quenched end, measured by Jominy end quenching method)  (2)

TABLE 3 Absolute Value of (Length after Groove Heat Treatment - Radius Width Ms Length before Change Change Point J1.5 J9 J11 Bend Heat Treatment) Amount Amount No. (° C.) (HRC) (HRC) (HRC) J9/J1.5 J11/J1.5 (mm) (mm) (mm) (mm) Present 1 421 45.6 37.2 33.5 0.82 0.73 0.015 0.011 0.005 0.017 Invention 2 444 45.5 38.0 35.1 0.84 0.77 0.015 0.003 0.008 0.021 Examples 3 425 48.1 35.4 32.3 0.74 0.67 0.020 0.015 0.003 0.011 4 395 49.3 41.0 38.5 0.83 0.78 0.010 0.003 0.006 0.019 5 421 50.0 36.9 34.6 0.74 0.69 0.010 0.018 0.004 0.014 6 433 45.2 37.2 32.5 0.82 0.72 0.005 0.007 0.006 0.022 7 432 44.4 35.1 33.1 0.79 0.75 0.015 0.013 0.006 0.016 8 419 47.8 34.8 33.1 0.73 0.69 0.015 0.022 0.002 0.012 9 409 48.1 35.4 32.4 0.74 0.67 0.020 0.023 0.003 0.014 10 433 48.4 35.6 33.3 0.74 0.69 0.015 0.023 0.002 0.013 11 403 50.2 40.7 38.5 0.81 0.77 0.010 0.005 0.006 0.021 12 412 51.1 39.1 36.9 0.77 0.72 0.025 0.014 0.006 0.016 13 399 49.3 41.6 38.7 0.84 0.78 0.020 0.006 0.008 0.021 14 435 46.7 36.4 34.2 0.78 0.73 0.015 0.012 0.004 0.015 15 442 47.7 35.4 32.5 0.74 0.68 0.010 0.017 0.003 0.011 16 426 48.2 38.1 35.2 0.79 0.73 0.010 0.018 0.004 0.013 17 435 45.2 34.9 32.1 0.77 0.71 0.020 0.014 0.005 0.014 18 408 49.2 40.0 37.8 0.81 0.77 0.020 0.005 0.006 0.022 19 431 47.8 34.3 32.3 0.72 0.68 0.020 0.020 0.004 0.011 20 409 45.3 37.5 35.5 0.83 0.78 0.025 0.003 0.007 0.022 21 388 46.5 39.4 36.4 0.85 0.78 0.030 0.005 0.006 0.024 22 421 48.7 38.6 35.8 0.79 0.74 0.025 0.008 0.005 0.011 23 413 50.2 40.1 36.7 0.80 0.73 0.015 0.007 0.007 0.016

Similarly, in Table 4, there are shown the measured Ms points of the steels of the comparative examples, Jominy hardenability as hardness J1.5 (HRC) at a distance of 1.5 mm from the quenched ends, hardness J9 (HRC) at a distance of 9 mm, or hardness J11 (HRC) at a distance of 11 mm, measured by the Jominy end quenching method, and the determined values of (J9/J1.5) and (J11/J1.5). The bends (unit: mm) after quenching of the above-described test pieces, the absolute values (unit: mm) of the differences between the lengths of the test pieces before and after the heat treatment, and the radius change amounts (unit: mm) and groove width change amounts (unit: mm) of the test pieces before and after the heat treatment, determined by the above-described method, are further shown.

TABLE 4

(The shaded portions fall outside the scope of claims.)

In the above-described invention examples Nos. 1 to 23, of which the composition ranges, excluding Fe and unavoidable impurities other than Ni and Mo, of the steel materials were those shown in Table 1, the bends of the test pieces after the heat treatment were able to be reduced to the small range of 0.005 to 0.030 mm, the absolute values of the differences between the lengths of the test pieces before and after the heat treatment were able to be further reduced in the small range of 0.003 to 0.023 mm, the radius change amounts before and after the heat treatment were able to be further reduced in the small range of 0.002 to 0.008 mm, and the groove width change amounts before and after the heat treatment were able to be further reduced in the small range of 0.011 to 0.024 mm by setting the Ms points at 388 to 444° C., which were 460° C. or less, and by suitably controlling hardenability measured by the Jominy end quenching method, thereby setting the values of (J9/J1.5) calculated by equation (1) in the range of 0.72 to 0.85 and setting the values of (J11/J1.5) calculated by equation (2) in the range of 0.67 to 0.78.

In contrast, in the above-described comparative examples Nos. 1 to 16, in which the composition ranges, excluding Fe and unavoidable impurities other than Ni and Mo, of the steel materials were those shown in Table 2, the remaining 14 examples excluding two examples No. 2 and No. 16 had the composition ranges falling outside those of the present invention. In the steels of the comparative examples Nos. 1 to 16, the values of (J9/J1.5) determined by equation (1) and the equation (2) from hardness measured by the Jominy end quenching method fall outside the range of 0.70 to 0.85 and the values of (J11/J1.5) fall outside the range of from 0.67 to 0.78. Among the steels of the comparative examples Nos. 1 to 16, nine examples have measured Ms points of more than 460° C. The comparative examples 1 to 16 have bends of the test pieces after the heat treatment, of 0.050 to 0.090 mm, all of which are greater than those of the steels of the invention examples. Further, in the steels of the comparative examples Nos. 1 to 16, any one or more values of the absolute value of the difference between the lengths of each test piece before and after the heat treatment, the radius change amount, and the groove width change amount before and after the heat treatment are greater than those of the steels of the present invention. Accordingly, none of the comparative examples had all of the bend of the test piece after the heat treatment, the absolute value of the difference between the lengths of the test piece before and after the heat treatment, the radius change amount and the groove width change amount before and after the heat treatment that were equivalent to those of the steels of the present invention examples.

The present invention example Nos. 1 to 23, of which the Ms points satisfy the scope of the claims of the present invention and the values of (J9/J1.5) and (J11/J1.5) satisfy the scope of the claims of the present invention, have generally low bends of the test pieces after the heat treatment, low absolute values of the differences between the lengths of the test pieces before and after the heat treatment, low radius change amounts and low groove width change amounts before and after the heat treatment, resulting in reduced heat treatment deformation, in comparison with the comparative examples. In addition, the steel materials of the present invention examples are subjected to heat treatment together with quenching for hardening a component, such as carburizing and quenching, thereafter tempered, and then used.

In view of the above, smaller heat treatment deformation after processing a steel material into a component and then performing heat treatment together with quenching for hardening the component, such as carburizing and quenching, can be achieved by the limitation of the steel constituents, the limitation of an Ms point as martensitic transformation start temperature, and the limitation of hardenability measured by the Jominy end quenching method, in the present invention. As a result, the steel material according to the present invention is a steel material that can be applied to components for power transmission, such as gears and shafts used in automobiles, industrial machines, and the like. 

1. A machine structural steel material with small heat treatment deformation, the steel material being a machine structural steel comprising in mass %: C: 0.20 to 0.30%; Si: 0.10 to 1.50%; Mn: 0.10 to 1.20%; P: 0.030% or less; S: 0.030% or less; Cr: 1.30 to 2.50%; Cu: more than 0% and 0.30% or less; Al: 0.008 to 0.300%; O: 0.0030% or less; N: 0.0020 to 0.0300%; and the balance Fe and unavoidable impurities, wherein the steel material comprising the steel has a martensitic transformation start temperature (Ms point) of 460° C. or less; a value of (J9/J1.5) calculated by following equation (1) using hardness J1.5 at a distance of 1.5 mm and hardness J9 at a distance of 9 mm from a quenched end of the steel material, as measured by Jominy end quenching method for the steel material, is in a range of from 0.70 to 0.85; and a value of (J11/J1.5) calculated by following equation (2) using hardness J1.5 at a distance of 1.5 mm and hardness J11 at a distance of 11 mm is in a range of from 0.67 to 0.78: (J9/J1.5)=(hardness at distance of 9 mm from a quenched end, as measured by Jominy end quenching method)/(hardness at distance of 1.5 mm from a quenched end, as measured by Jominy end quenching method)  Equation (1); and (J11/J1.5)=(hardness at distance of 11 mm from a quenched end, as measured by Jominy end quenching method)/(hardness at distance of 1.5 mm from a quenched end, as measured by Jominy end quenching method)  Equation (2).
 2. The machine structural steel material according to claim 1, wherein the steel material further comprises, in mass %, one or two of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.50%.
 3. The machine structural steel material according to claim 1, wherein the steel material further comprises, in mass %, one or two of Ti: 0.020 to 0.200% and Nb: 0.02 to 0.20%.
 4. The machine structural steel material according to claim 1, wherein the steel material further comprises, in mass %, one or two of Ni: 0.20 to 3.00% and Mo: 0.05 to 0.50%, and one or two of Ti: 0.020 to 0.200% and Nb: 0.02 to 0.20%. 